1. Field of the Invention
The present invention relates to the removal of mineral pollutants from water, and more particularly, to an improved method for recovery of spent regenerants and removal of mineral salts therefrom.
2. Description of the Prior Art
Deionization processes employing weak acid and weak base resins are per se known to the art. Two very real advantages in using these weak resins are the achievement of very high regeneration efficiencies and a high theoretical loading capacity. Both types of weak exchangers can easily and effectively be regenerated to high levels by employing amounts of regenerant only slightly in excess of stoichiometry.
An improved process for the removal of mineral pollutants from water utilizing weakly acidic cation resin and weakly basic anion resin is disclosed in U.S. Pat. No. 3,700,592. The cation resin is regenerated with a chelating agent while the anion resin is regenerated with an organic solution of a base. The spent cation regenerant as a metal chelate is treated with carbon dioxide to precipitate a metal carbonate and free the chelate regenerant for recycle. The metal carbonate is then combined with the spent anion regenerant (amine salt) at elevated temperatures to precipitate the metal salt and recover carbon dioxide and amine regenerant for recycle.
An improved recovery process which eliminates separate treatment of the spent regenerant before reconstituting the mineral salt is disclosed in a copending application Ser. No. 496,619, filed concurrently herewith, entitled "Solid Mineral and Regenerant Recovery for Ion-Exchange Resins". In the improved process, a basic spent cation regenerant is combined with an acidic spent anion regenerant to form a weak dissociable complex of the regenerants and to reconstitute the mineral salts. The spent cation and anion regenerants are combined either in a continuous or batch fashion by blending near stoichiometric quantities of the materials. This stoichiometric adjustment is based on the amount of the two streams necessary to reconstitute the mineral salts removed from the spent ion-exchange resins. The regenerant streams may be either aqueous solutions or organic solvent water-solutions.
Among the cation resin regenerants disclosed in the aforementioned copending applications are metal complexing agents such as .beta.-diketone chelating agents such as 1,3-cyclohexanedione. Although these chelating agents are extremely effective regenerants, they suffer to some extent from loss through dimerization at elevated temperature under alkaline conditions such as encountered during the distillation recovery of the amine regenerant. Substitution with sterically hindering groups such as aliphatic groups in the 5- or 2-position of cyclohexanedione prohibits the dimerization reaction. However, such substitution decreases the water solubility of the chelate and the resulting metal chelate.